1. Field of the Invention
Embodiments of the present invention generally relate to a magnetic read sensor. Specifically, embodiments of the invention relate to the composition of a pinned layer in a magnetoresistive read sensor.
2. Description of the Related Art
Modern computer systems typically include a hard drive which may be used as mass storage for the computer system. Information in the hard drive is typically stored as magnetic charge on one or more magnetic disks within the hard drive. To read the information, the hard drive includes a magnetic read sensor which senses the magnetic charge as the magnetic disks spin beneath or above the magnetic read sensor.
Modern magnetic read sensors typically include several layers of material deposited on a substrate. As the magnetic read sensor passes over an area of a magnetic disk in which a magnetic charge is present, the magnetic charge may induce a detectable change in the layers of material in the magnetic read sensor. For example, during a read operation, a current may be applied to the layers of material in the magnetic read sensor. The current applied to the layers of material may create a voltage across the layers of material which is proportional to the resistance of the layers of material. As the sensor passes over a magnetic charge on the magnetic disk, the magnetic charge may cause a change in the combined resistance of the layers of material (e.g., an increase or decrease in the resistance of the layers of material). The stored magnetic charge may then be measured via a corresponding change in the voltage across the layers of material (e.g., a corresponding increase or decrease in the voltage resulting from the current applied to the layers of material).
One type of magnetic read sensor is a spin valve. A spin valve typically contains a free layer, an anti-parallel (AP) pinned layer, and an antiferromagnetic (AFM) pinning layer. The pinned layer has a magnetic moment which is pinned (fixed) in a given direction by the pinning layer and which typically is not easily rotated by external magnetic fields. The free layer has a magnetic moment which is not pinned and can be easily rotated by external magnetic fields. The resistance of the GMR sensor is typically given by the formula R=Rμμ+(Rμo−Rμμ)(1−cos Θ/2. Here Rμμ is the sensor resistance when pinned and free layer moments are parallel (Θ=0), Rμo is the sensor resistance when the pinned and free layer moments are anti-parallel (Θ=π), and Θ is the angle between free and pinned layer and Rμμ<<Rμo. Thus when the magnetic moment of the free layer is parallel to the magnetic moment of the pinned layer, the resistance of the read sensor may be decreased, and when the magnetic moment of the free layer is not parallel to the magnetic moment of the pinned layer the resistance of the read sensor may be increased. As described above, the change in resistance of the magnetic read sensor may be used to read the pattern of magnetic transitions present on a magnetic disk.
Several factors may affect the manufacture and operation of the magnetic read sensor. For example, high exchange coupling between the AFM layer and pinned layer which is typically quantified by a high magnetic pinning field (Hp) is typically desired for pinning the pinned layer. When the pinning field is high, the orientation of the magnetic moment of the pinned layer may not be easily affected by other magnetic fields, thereby limiting noise, ensuring linear behavior of the magnetic read sensor, and maintaining a large change in resistance of the magnetic read sensor during operation.
Another aspect of the pinned layer is the coercivity (Hcp) of the pinned layer. The coercivity affects how susceptible the magnetic moment of the pinned layer is to being permanently modified, e.g., due to inadvertent heating during manufacturing of the magnetic read sensor or due to a high magnetic field being applied to the layer inadvertently during manufacture. Typically, a low coercivity is desired so that such incidental aspects of the manufacturing process are not detrimental to the pinned magnetic moment of the pinning layer.
Yet another aspect of the pinned layer is the blocking temperature (Tb) of the pinned layer. Typically, as the temperature of a pinned layer is increased, the strength of the magnetic pinning field of the pinned layer decreases. The blocking temperature is typically defined as the temperature at which the magnetic pinning field of the pinned layer is reduced to zero. In order to maintain a high pinning field (and thus, an increased resistance change/sensitivity of the magnetic read sensor) across a variety of temperatures, a high blocking temperature is typically desired. A high blocking temperature may also prevent damage to the pinned layer if the pinned layer is inadvertently heated during manufacturing of the magnetic read sensor. Thus, a magnetic read sensor is desired which provides a pinned layer with a high pinning field, low coercivity, and high blocking temperature.
Accordingly, what is needed is a magnetic read sensor with an improved pinned layer.